Looks like something like this adds about 1.6 oz so should still be under the ~4oz total add allowed.
https://www.glockstore.com/Double-Di...sten-Guide-Rod
ETA:
looks like the Thug Plug would add another ~2.3 oz which would get you awful close to that 4oz limit if you combined the two.
https://www.glockstore.com/Thug-Plug...=6&custcol8=16